Corrosion resistant gasket for aircraft

ABSTRACT

A corrosion resistant gasket for aircraft, and more particularly to encapsulation of a conductive mesh in a pliable fluorosilicone compound that will not become bonded to mating surfaces and will also migrate upon compression during installation to the threads of attaching hardware utilized between the instrument, antenna, and aircraft structure, thereby reducing corrosion through the attaching hardware, by providing a hermetic seal around the periphery of the envelope of the attached device. In addition, a flexible seal comprising a fluorosilicone, a silicone, and silica is disclosed. The flexible seal provides a fluid-resistant barrier and is readily installed and removed from electrical and/or mechanical components in conjunction with the use of a conductive corrosion resistant gasket described in U.S. Pat. No. 5,791,654.

RELATED APPLICATIONS

This is a divisional application of continuation-in-part applicationSer. No. 09/549,941, which is a continuation-in-part ofcontinuation-in-part application Ser. No. 08/861,179 filed May 21, 1997,which is a continuation-in-part of application Ser. No. 08/602,550,filed Feb. 20, 1996, abandoned, which is a continuation-in-part ofapplication, Ser. No. 08/356,983 filed Dec. 16, 1994, abandoned, whichis a continuation-in-part of application, Ser. No. 08/233,869 filed Apr.26, 1994, abandoned, which is a continuation-in-part of application Ser.No. 07/932,098 filed Aug. 19, 1992,abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a corrosion resistant gasket for aircraft, andmore particularly to encapsulation of a conductive mesh in a pliableuncured, uncatalyzed fluorosilicone compound having a durometer of 40 orless that will not become bonded to mating surfaces and will alsomigrate upon compression during installation to the threads of attachinghardware utilized between the instrument, antenna, and aircraftstructure, thereby reducing corrosion through the attaching hardware, byproviding an outer bead around the periphery of the envelope to providea hermetic seal of the attached device. An additional component is theincorporation of an outer flexible seal.

Present methods of providing coupling between mating surfaces inaircraft having aluminum structures were limited to the uses of curedelastomer gaskets, metallic gaskets using sealants, or a multiple use ofcorrosion inhibitors and plating. The cured elastomer gaskets wouldallow moisture between the mating surfaces and tended to becomebonded/adhere or retain a memory under compression to the two surfacesafter a period of time and temperature cycling. The metallic gasketsalso had a permanent bonding problem due to the application of adhesivesto reduce the moisture ingress between the two surfaces. Both elastomerand metallic gaskets tended to shift the frequency of antennainstallations due to the gap they created between the two matingsurfaces, causing a shift in the VSWR of the antenna. The use of thecorrosion resistant compounds and sealants creates a time consumingprocess in application and removal and tend to crack during structureflexing, thereby allowing moisture to ingress between the matingsurfaces and causing a breakdown of the inhibitors.

Also, most gaskets presently used have a base material so dissimilar toaluminum that they thereby cause galvanic corrosion, rather than preventit, due to the fact that they cannot provide a hermetic seal bythemselves and require the use of an outside sealant which when used inhigh vibration areas or under flexing conditions tends to crack andthereby introduces an electrolyte that creates a galvanic cell.

Other alloy meshes such as Monel, a nickel plated copper alloymanufactured by The Chomerics Co. of Woburn, Mass., embedded in siliconegels are dissimilar metals with respect to aircraft structure oraircraft antenna base, such that they are subject to corrosionthemselves or cause additional corrosion through galvanic corrosion whenexposed to certain unfavorable environments. Also, it has been proven,both by lab tests and in service applications, that the use of siliconeon aircraft in areas where the silicone is either exposed to jet fuel orjet fuel vapors, the silicone deteriorates and consequently thecorrosion protection is jeopardized in the use of aircraft applications.

Accordingly, there exists a need in the art for electrical and/ormechanical seals that are easily installed and replaced and that arealso impervious to fluids. More specifically, a need exists for afluid-resistant, self-adhesive, and flexible insulation seal that isstable over a large range of temperatures. The present invention seeksto fulfill these needs and provides further benefits.

SUMMARY OF THE INVENTION

The present invention provides a gasket which prevents the ingress ofmoisture and or other contaminates between the surfaces of aluminumcausing corrosion or galvanic corrosion in this area. The gasket isconstructed so that it eliminates present electrical bonding methodsthrough the attaching screws and provides a positive bond through thestructure to the base of the instrument, antenna, and/or aircraft skinlap joints, electrical receptacle outlets, waste outlets, lavatoryinstallations and galley installations. Airplane structural accesspanels such as wing fuel access panels is one aspect. The presentinvention provides a flexible seal forming a curable compositionincluding a fluorosilicone, a silicone, and silica. In one embodiment,the flexible seal is formed by curing the composition after itsinstallation on or about an electrical and/or mechanical componentsought to be protected. In another embodiment, the flexible seal of thepresent invention is formed by curing the curable composition byapplying heat. In some instances, the curable composition wrapped withthe fusible tape may be cured from heat generated by the system in whichthe sealed component is a part. Besides providing electrical bondingthrough surface to surface contact, eliminating all aspects ofcorrosion, the gasket is capable of reducing corrosion through theattaching hardware by migrating the insulating properties of the uncuredunvulcanized fluorosilicone material of the gasket onto the threads ofthe attaching hardware and forms an outer bead of the fluorosiliconecompound around the periphery of the installation upon compression.Although the gasket is shown in certain illustrative embodiments hereinin aircraft applications, it is also useful in marine applications wheresalt water corrodes aluminum or steel installations and wheremaintaining electrically conductive properties between mating surfacesis a serious problem.

By using the present flexible gasket comprising silver-plated strandedaluminum when low electrical bonding resistance is required in certainaircraft installations requiring less than 0.02 milliohm encapsulated inan uncured unvulcanized fluorosilicone, application time is reduced, aswell as removal and elimination of structural and component damageduring removal. Since the fluorosilicone compound provides a hermeticseal under high vibration, flexing conditions, aerodynamic conditions ofup to 0.8 Mach, and provides a seal for internal aircraft pressure ofover 30 P.S.I., there is no fear of an introduction of either anoutside, or inside introduction of an electrolyte that would create agalvanic cell. For the purposes of cost reductions in applications thatdo not require extremely low bonding resistance requirements, and onlyneed to be less than 1 milliohm, then the aluminum woven metallic meshor expanded aluminum screen can be of the same surface structure typecurrently used in the manufacture of aircraft structures. The expandedaluminum screen is favored over the metallic woven mesh for airplaneapplications. The reason for the preference of the expanded aluminumscreen for airplane applications is the pliability of the expandedaluminum which is made from an expanded metal foil is one of severalmesh (or open-area) materials used in eliminating interference. It ismade from solid foil gauge metals to precision tolerances. The processinvolves a shaped tool lancing and stretching a ductile metal in onemotion. The resulting holes are diamond-shaped with a large variety ofhole sizes. Dimensions range from a {fraction (1/32)}″ in the long wayof the diamond (LWD) to approximately ½Δ LWD. In shielding applicationsan LWD of ⅛″ or less is most common, although different shape holes canalso be created. Material options include selvage edge, solid sectionsand annealing. Upon compression the screen will make contact with thetwo mating surfaces to provide electrical continuity, but will not causeany pits into the airplane structure as will the woven metallic meshwith the harder durometer. This has been observed on in-service airlinefield tests and discovered after a period of two years with periodicinspections using the woven metallic mesh.

By encapsulating the woven mesh or expanded aluminum screen with afluorosilicone compound such as Dow Corning LS40 or GE FSE2120, or anyother fluorosilicone compound that is not vulcanized and catalyzed theend product can be achieved. The fluorosilicone compound is best appliedin a sheet form to a precut or die cut configuration of the metallicmesh or screen the configuration of the device to be applied to. Theexpanded aluminum screen should not exceed more than 20% of thethickness of the fluorosilicone compound. Application of the compound tothe mesh can be accomplished by placing a layer of the compound on awarm pad and placing the aluminum screen or mesh on top. Then by the useof a vacuum bag, cover the heated gasket and drawing vacuum, thefluorosilicone compound will flow through the mesh, and also fill allthe installation cut-outs for the attaching screws.

FIG. 8 shows a gasket that has an inner elastomer in the die cut for theattaching hardware that provides corrosion protection to the screws andnut plates or fasteners. FIG. 8 shows the use of two layers of thescreen or expanded aluminum mesh of FIG. 7 and die cut of the same tothe desired configuration and the insertion of the uncured elastomerbetween the two conductive layers. This eliminates the need for specialtype release liners and can be packaged for application in any type ofplastic or paper container. When applied under pressure the two aluminummesh 32 meet and form the electrical contact needed between the twomating surfaces and the elastomer 31 squeezed between the openings andouter periphery of the exposed mesh to encapsulate the mesh and providea hermetic seal. The desired thickness of elastomer 31 is approximately0.20 inch with the expanded height of the mash being 0.17 inch.Depending upon the depth of the groove or channel, the described initialbuildup can be multiple layers to form any particular height. At thesame time that the squeegee action of the elastomer is taking place, ifthere is any moisture on either of the mating surfaces, it is expelledby the application of the elastomer.

The present gasket provides a hermetic seal between two mating surfacesand provides corrosion protection between those surfaces that thepliable material is in contact with, including attaching hardware whenthe compound is migrated to the threads during installation.

The internal mesh or screen provides a positive electrical bond toreduce lightning strike and improve antenna performance such aseliminating “P” static build up when under compression of the base andstructure. The outer flexible seal is directed to an electrical andmechanical seal that is easily installed, repaired, and replaced.

In one embodiment, the flexible seal is a curable composition thatincludes a fluorosilicocne, a silicone and silica. As used herein, theterm “curable composition” refers to a composition that includescomponents which, through their chemical functionality, may bepolymerized and/or crosslinked to one another or other suitablyfunctionalized components in the composition. In another embodiment, theflexible seal is a cured composition resulting from the application ofheat to curable composition comprising a fluorosilicone, a silicone, andsilica. The flexible seal of the present invention can be installed onor about an electrical and/or mechanical component to provide afluid-resistant barrier to protect the component from water andhydrocarbon liquids such as fuels, oils, and solvents.

The curable silicone composition includes a combination of afluorosilicone which imparts solvent resistance to the composition and asilicone which imparts flexibility and resilience to the composition.The hardness of the seal may be modified by varying the amount offluorosilicone component in the composition. Increasing the amount ofthe fluorosilicone present in the composition generally increases thehardness seal. The relative amounts of the fluoroslicone and silicone inthe composition may be varied to provide flexible seals having a rangeof harnesses (i.e., flexibility and having resistance to specificfluids. In a preferred embodiment, the fluorosilicone is present in thecomposition in an amount from about 5% to about 20% by weight of thetotal composition, the silicone is present in the composition in anamount from about 10% to about 30% by weight of the total composition,and the silica is present in the composition in an amount from about 50%to about 85% by weight of the total composition.

The curable composition noted above includes both a fluorosilicone and asilicone. As used herein, the term “silicone” refers to apollydiorganosiloxane, which is a polymer having a diorganosiloxanegroup (i.e., —SiR₂O—) as the repeating unit. The silicones useful in thepresent invention include homopollymers, such as polydimethylsiloxane,(CH₃)₃Si—O((CH₃)₂Si—O)_(n)—Si(CH₃)₃), co-polymers, such aspolymethyldiphenysiloxane,(C₃)₃Si—O((CH₃)₂Si—O)_(n)(Ph₂Si—O)_(n)(Ph₂SI—O)_(m)—Si(CH₃)₃), and theirderivatives, where n and m are integers representing the number ofrepeating diorganosilixane units present in the polymer. Thepolydiorganosilixanes useful in the present invention may be representedby the following formula:

T—SiR₂—(O—SiR2)_(n)—OsiR₂—T

where the subtituent R is independently selected from a hydrogen atom,an alkyl group such as a methyl group, a fluorinated alkyl group such asa trifluoropropyl group, an alkenyl group such as a vinyl group, and anaryl group such as phenyl group, substituent T is a terminating group,such as one of the substituents noted above for R, or another functionalgroup to facilitate the polymerization and/or crosslinking (i.e.,curing) of one polydiorganosilixane repeating units present in thepolydiorganosiloxane. Suitable functional groups, preferably vinylgroups, hydrolyzable groups, condensable groups, such as hydroxy groups,alkoxy groups, and halogen groups, including chloride and bromide. Thepolysiloxanes useful in this invention may also be polymerized and/orcrosslinked through hydrosilytion where two siloxanes couple throughreaction of an alkenyl group present as a substituent on one siloxaneand silicone hydrogen group (i.e., Si—H) present on another siloxane.

Suitable silicones of the curable composition of this invention includepolydiorganosiloxanes. Preferred polydiorganosiloxanes includepolydimethylsiloxane, polyddimethyl vinylmethylsiloxanes, polyiphenyldmethylsiloxanes, and their derivatives such as, for example,dimethylvinyl terminated polyldimethyl methylvinlsiloxane.

The curable composition of the invention also includes silica as areinforcing filler to improve the physical strength of the curablecomposition and the flexible seal. Suitable forms of silica includequartz, amorphous silica, trimethylated silica, fumed silica,precipitated silica, and combinations of these forms.

The curable composition may also include a curing catalyst to effect thepolymerization and/or crosslinking of the silicones presentcompositions. Suitable polymerization and/or crosslinking of thesilicones present compositions. Suitable catalysts are known and includecatallysts that accelerate the processes of vinyl polymerization,silannol condensation, and hydrosilylation. In addition, the layup ofthe curable composition on an electrical and/or mechanical component mayreadily include, for example, a layer of wire mesh to afford electricaland magnetic shielding to the component.

As noted above, the flexible seal of this invention may be prepared byapplying heat to the curable composition after the composition has beeninstalled on or about an electrical and/or mechanical component. Thecomposition may be cured by heating at a temperature of about 250° F. toabout 350° F. for about 20 to about 45 seconds. In a typicalapplication, heat may be applied through the use of a heat gun.

The flexible seal has been found to protect components from both dry andwet environments and provide electrical insulation and other desiredsealing qualities over a temperature range from about −65° F. to about500° F.

The flexible seal has been determined to be impervious to water. Infact, when the curable composition is applied to a wet surface, themigration of entrapped water away from the component and out of the sealhas been observed. The flexible seal has also been determined toeffectively insulate components from other fluids including, forexample, jet fuel, hydraulic fluid, oil, and organic solvents such asmineral spirits, hexane, toluene, xylene, and the like. The flexibleseal also withstands immersion in such fluids up to altitudes of about100,000 feet, maintaining the integrity of the seal including electricalinsulation and corrosion inhibition.

In contrast to the material described e.g. in U.S. Pat. No. 4,900,877,the insulating material must be made from a fluorosilicone material thatmay be made from a commercial colloidal substance or from a thixotropicfluorosilicone material that is uncatalyzed, unvulcanized, has no watercontainment in it's make up, and free from bubbles and voids so as toprovide a hermetic seal from the outside environment as well as tocontain internal aircraft contaminates from migrating from the connectororifice to the outer edge of the instrument or antenna that the gasketis mated between. Silicone or fluorosilicone gels have a certain amountof water in their makeup. This is an undesirable feature that causesinternal corrosion by providing an electrolyte between the mesh andstructures when the gel is displaced under compression. This has beenproven during extensive in-service airplane field tests. The other notedundesirable effect was the migration of the gel and internal siliconeoils to the outer surrounding surface under compression. This siliconecontamination then reduces good paint adhesion. The conductive mesh orscreen must also have a minimum hole opening of 0.065 inch with aminimum of 8 holes per inch with the X monofilament dimension height tobe twice that of the Y dimension width so as to provide pliableconformity to the aircraft contour and provide surface contact to themating surfaces at fifteen inch pounds of compression. The conductivematerial must also be of a low resistance so as to provide the integrityneeded for system performance and against the hazards encountered withlightning strike. This precludes the use of any type of material that isidentified as E.M.I. shielding material, since these types of materialsconsist of higher resistive materials such as Monel that provide againsthigh frequency penetration or High Intensity Radiated fields (H.I.R.F.)protection.

With the present gasket, application time is minimal and removal time isequal to the application time, and further, structural and componentdamage is eliminated during removal.

The present gasket further protects the installation in harshenvironments of aircraft fluids, altitude immersions to 75,000 feet,vibration, structural flexing, and temperatures of −65° to 450° F.

DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to embodimentsthereof shown for purposes of illustration in the accompanying drawingswherein:

FIG. 1 is an exploded view of the layup of layers of a sandwichstructure used to form different geometry gaskets;

FIG. 2 is a gasket having a geometry suitable for a marker beaconantenna subsequent to assembly and die cutting of the sandwich structurelayup of FIG. 1;

FIG. 3 is an alternative die cutting of the sandwich structure layup ofFIG. 1 having a geometry suitable for an instrument gasket for mountinga total air temperature instrument to an aircraft structure matingsurface;

FIG. 4 is an exploded view of a gasket layup according to FIG. 1,utilizing a mesh in the sandwich structure;

FIG. 5 is an exploded view of a gasket layup according to FIG. 1,utilizing a knit in the sandwich structure;

FIG. 6 is illustrative of the method of assembly of an aircraft antennato an aluminum outer surface aircraft skin subsequent to removal ofrelease liners from the layup of FIG. 5 and prior to compression of theaircraft antenna to the aircraft surface.

FIG. 7 is illustrative of an expanded aluminum mesh showing strand holeopening; and

FIG. 8 is illustrative of a further embodiment of the invention where anuncured elastomer is inserted between two conductive layers.

FIG. 9 is an illustrative drawing of the flexible seal around theconductive gasket.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Turning now to FIG. 1, wherein an aluminum monofilament is woven into amesh or knot electrically conductive structure 1 which is seen to bepositioned intermediate adjacent layers of compressible fluorosiliconecompound 2; electrically conductive structure 1 and adjacent layers ofcompressible fluorosilicone compound 2 are then seen to be furthersandwiched between liners 3 applied to each side of the outer surfacesof layers of compressible fluorosilicone compound 2 to protect layersand the fluorosilicone compound 2 mesh 1 structure against contaminantsprior to use. The layers 1, 2, and 3 of the sandwich assembly of FIG. 1are assembled together and die cut out into the geometry of an antennagasket 14, as shown in FIG. 2. A plurality of holes 12 are suitablepositioned in antenna gasket 14 for attaching hardware 22 utilized toattach the antenna 20 (seen in FIG. 6) to the aluminum aircraft skinsurface 30.

Gasket 33, of FIG. 3 is die cut from the sandwich assembly layup of FIG.1 to a geometry suitable for mounting an aircraft instrument to aircraftstructure. Gasket 33 includes suitable holes 35 for mounting hardware(not shown) for fastening the instrument down on gasket 33 to theaircraft structure (not shown).

Release liners 3 of FIGS. 1, 4, and 5 is of the type used to protectadhesive transfer on labels. Mesh 5 of the sandwich layup of FIG. 4 is atight weave, 0.003 in. of larger, plated with a 0.001 inch silver metalon a 0.003 mil. aluminum monofilament. In the sandwich layup of FIG. 5,instead of a mesh 4, as in FIG. 4, a knit in the form of a sock 4 isutilized, so that a spacer or non-conductive material can be inserted asshown at 40 between the outer surfaces of sock 4, so that as to make theinserted spacer or non-conductive material conductive and at the sametime prevent corrosion between two adjoining parts.

Where a weave sock 4 includes therewithin a non-conductive spacerinserted at 40 between the sides of weave sock 4 and utilized in thegasket assembly of FIG. 6 for fastening antenna 20 down to aluminumaircraft skin surface 30, a conductive corrosion-proof gasket resultswhich eliminates the previous dielectric spacer effect that reducedantenna performance, thereby providing an electrically conductivebonding between the two mating surfaces.

Prior to compression of the gasket assembly of 6 by tightening down ofmounting hardware 22 upon aircraft antenna 20 to aluminum aircraft skinsurface 30, protective release liners 3 (shown in FIG. 5) are removed,and in the compression step, fluorosilicone compound layers 2 arecompressed to result in a gasket having a thickness of the centerconductive woven silver-plated aluminum monofilament structure ofbetween about 0.31 and 0.010 inches. The present gaskets eliminate anyelectrolyte between two mating components after compression and achieveone electrical bond between the two mating surfaces. Since the gasketprovides a hermetic seal by itself, the need for additional sealantscommonly used by other types is not required with the fluorosiliconecompound or thixotropic gasket with a conductive inner mesh.

FIG. 7 shows the expanded aluminum mesh with the base materialthickness, height or “X” dimension and the strand width or “Y” dimensionin the expanded form to show the strand hole opening, or “SHO” of theelectrically conductive material 1 also known as the long way of thediamond (LWD) in commercial applications; with the fluorosiliconecompound 2; being sandwiched together with the liner 3.

FIG. 8 as hereinbefore described is similar to the embodiment of FIG. 7with the deletion of the liner and the addition of a further aluminummesh.

FIG. 9 is shown as the top view of a typical wing access fuel panelgasket where the outer edge of the flexible seal can be added to any ofthe structures of FIGS. 1-8 hereinbefore described. The FIG. 9construction is similar to the structure of FIG. 1 with the exception ofadding the flexible seal to the inner and outer periphery of thestructure. This is shown where the top layer 1 is a conductive mesh withan inner layer of fluorosilicone compound 2 between the top and bottomlayer 1. The flexible seal 3 is placed between the conductive layers of1 and flush to the fluorosilicone compound 2. The outer edge of theflexible seal 3 is extended away from the inner and outer conductivemesh 1 as required to form a fillet seal around the housing and accesspanel.

What is claimed is:
 1. A method of electrically conductively bondingopposing mating surfaces comprising the steps of: providing a layer offluorosilicone between a pair of electrically conductive woven members;and then compressing opposing mating surfaces of said pair ofelectrically conductive woven members together thereby electricallyconductively bonding said opposing mating and providing a hermetic sealbetween said opposing mating surfaces.